ALTHOUGH Cu(II) is not known to form π complexes with arenes in homogeneous solution, benzene1,2 and methyl substituted benzenes3 can complex with partially dehydrated Cu(II) ions on a montmorillonite surface through donation of π electrons. Benzene gives rise to two types of coloured complexes, type I (green) and type II (red), which exhibits an infrared spectrum suggesting loss of the aromatic character of benzene. Both forms are interconvertible by varying the moisture content of the system. With methyl substituted benzenes only type I complexes, where aromaticity is retained, are formed. Similarly, the adsorption of benzene and alkyl substituted benzenes on Ag(I)-montmorillonite4 and of phenol on Cu(II)- and Ag(I)-hectorite5 gives rise to type I complexes. With other inorganic cations little or no direct organic-cation interaction is observed. Here we deal with the adsorption of thiophene on Cu(II)-, H-, Na- and Ca-montmorillonite. Thiophene has been chosen because it has aromatic character and contains both π and free p electrons capable of interacting with the exchange cation.